In drilling a borehole (or wellbore) into the earth, such as for the recovery of hydrocarbons or minerals from a subsurface formation, it is conventional practice to connect a drill bit onto the lower end of an assembly of drill pipe sections connected end-to-end (commonly referred to as a “drill string”), and then rotate the drill string so that the drill bit progresses downward into the earth to create the desired borehole. In conventional vertical borehole drilling operations, the drill string and bit are rotated by means of either a “rotary table” or a “top drive” associated with a drilling rig erected at the ground surface over the borehole (or, in offshore drilling operations, on a seabed-supported drilling platform or suitably-adapted floating vessel).
During the drilling process, a drilling fluid (also commonly referred to in the industry as “drilling mud”, or simply “mud”) is pumped under pressure downward from the surface through the drill string, out the drill bit into the borehole, and then upward back to the surface through the annular space between the drill string and the wellbore. The drilling fluid, which may be water-based or oil-based, is typically viscous to enhance its ability to carry borehole cuttings to the surface. The drilling fluid can perform various other valuable functions, including enhancement of drill bit performance (e.g., by ejection of fluid under pressure through ports in the drill bit, creating mud jets that blast into and weaken the underlying formation in advance of the drill bit), drill bit cooling, and formation of a protective cake on the borehole wall (to stabilize and seal the borehole wall).
Particularly since the mid-1980s, it has become increasingly common and desirable in the oil and gas industry to drill horizontal and other non-vertical boreholes (i.e., “directional drilling”), to facilitate more efficient access to and production from larger regions of subsurface hydrocarbon-bearing formations than would be possible using only vertical boreholes. In directional drilling, specialized drill string components and “bottom hole assemblies” are used to induce, monitor, and control deviations in the path of the drill bit, so as to produce a borehole of desired non-vertical configuration.
Directional drilling is typically carried out using a “downhole motor” (alternatively referred to as a “drilling motor” or “mud motor”) incorporated into the drill string immediately above the drill bit. A typical downhole motor includes several primary components, as follows (in order, starting from the top of the motor assembly):                a top sub adapted to facilitate connection to the lower end of a drill string (“sub” being the common general term in the oil and gas industry for any small or secondary drill string component);        a power section comprising a positive displacement motor of well-known type, with a helically-vaned rotor eccentrically rotatable within a stator section;        a drive shaft enclosed within a drive shaft housing, with the upper end of the drive shaft being operably connected to the rotor of the power section; and        a bearing assembly (which includes a mandrel with an upper end coupled to the lower end of the drive shaft, plus a lower end adapted to receive a drill bit).        
In drilling processes using a downhole motor, drilling fluid is circulated under pressure through the drill string and back up to the surface as in conventional drilling methods. However, the pressurized drilling fluid exiting the lower end of the drill pipe is diverted through the power section of the downhole motor to generate power to rotate the drill bit.
In directional drilling, the path of the drill bit is deviated in a desired direction by means of a bent housing or a bent sub, typically disposed between the power section and the bearing assembly of a downhole motor. Bent subs and bent housings serve the same purpose, and in general terms differ only in that a bent housing is adapted to accommodate a drive shaft through its central bore. Although bent subs and bent housings may be fashioned with a fixed bend angle, it is commonly advantageous for a bent housing or bent sub to comprise an assembly of components whereby the bend angle is adjustable between being zero and some maximum bend angle.
Examples of known types of adjustable bent housings and bent subs may be seen in U.S. Pat. No. 5,125,463 (Livingstone et al.), U.S. Pat. No. 5,343,966 (Wenzel et al.), U.S. Pat. No. 6,515,901 (Falgout, Sr.), U.S. Pat. No. 6,550,818 (Robin), and Patent Publication No. US 2007/0095575 (Johnson et al.).
A typical adjustable bent housing comprises first and second tubular members separated by a tubular sleeve (or “adjustment ring”, as this element will be referred to herein), arranged in a generally end-to-end configuration and disposed about an internal tubular element of some type. A primary distinguishing feature of a typical adjustable bent housing is that each end of the adjustment ring is engageable with an end of either the first or second tubular member such that the centroidal axis of the adjustment ring is not coincident with the centroidal axis of either the first or the second tubular member, but is in each case angularly offset by a selected offset angle. As described in further detail later herein, this allows for angular adjustment of the first tubular member relative to the second tubular member so as to orient the centroidal axes of the first and second tubular members at a selected bend angle between zero and the sum of the offset angles (or twice the offset angle if, as is typically the case, the same offset angle is used at each end of the adjustment ring).
In one exemplary configuration of an adjustable bent housing, the first tubular member has a first end with a planar annular first end face, a second end, and a centroidal axis. The plane of the first end face of the first tubular member is perpendicular to the centroidal axis thereof. The second tubular member has a first end, a second end, and a centroidal axis. The first end of the second tubular member defines a first clutch profile, the general plane of which is offset from a plane perpendicular to the centroidal axis of the second tubular member by an offset angle φ (phi).
The adjustment ring has a first end, a second end, and a centroidal axis. The first end of the adjustment ring has a planar annular first end face, the plane of which is offset from a plane perpendicular to the centroidal axis of the adjustment ring by offset angle φ. In the assembled bent housing, the first end face of the adjustment ring is matingly engageable with the planar annular end face of the first end of the first tubular member.
The second end of the adjustment ring defines a second clutch profile, the general plane of which is perpendicular to the centroidal axis of the adjustment ring, and which in the assembled bent housing is engageable with the first clutch profile at the first end of the second tubular member, for rotationally locking the second tubular member relative to the adjustment ring.
The first and second tubular members and the adjustment ring are disposed about an internal tubular member such that a first end of the internal tubular member extends into the first tubular member, and a second end of the internal tubular member extends into the second tubular member. The internal tubular member has a first centroidal axis associated with the first end of the internal tubular member, and a second centroidal axis associated with the second end of the internal tubular member, with the first and second centroidal axes being angularly offset, and intersecting in a medial region of the internal tubular member.
The first end of the internal tubular member has external threading engageable with internal threading in the first end of the first tubular member, such that in the assembled housing, the first centroidal axis of the internal tubular member will be coincident with the centroidal axis of the first tubular member.
The second end of the internal tubular member has external threading engageable with internal threading in the first end of the second tubular member. However, the internal threading in the first end of the second tubular member is concentric not with the centroidal axis of the second tubular member but is instead concentric with a non-centroidal secondary axis (or “skew axis) offset from the centroidal axis of the second tubular member, such that in the assembled housing, the skew axis will be coincident with the second centroidal axis of the internal tubular member. The magnitude of the offset between the skew axis and the centroidal axis of the second tubular member will typically be equal to offset angle φ.
The adjustment ring is longitudinally and non-rotatingly slidable along a medial portion of the internal tubular member, typically by means of a splined connection between these two components.
The angular variance between the centroidal axes of the first and second tubular members (herein referred to as the bend angle θ (theta) of the bent housing assembly) can be adjusted by rotating the first tubular member (typically counterclockwise) relative to the internal tubular member so as to separate the first faces of the first tubular member and the adjustment ring, thus allowing sliding movement of the adjustment ring along the internal tubular member toward the first tubular member so as to disengage the first and second clutch profiles. The bend angle can then be adjusted by rotating the adjustment ring so as to change the angular relationship between the clutch profiles, which can then be re-engaged by sliding the adjustment ring back toward the second tubular member. The first tubular member is then rotated (typically clockwise) to tighten its first end face against the first end face of the adjustment ring.
Because the adjustment ring is non-rotatable relative to the inner tubular member, rotation of the adjustment ring relative to the second tubular member results in the same angular rotation of the inner tubular member relative to the second tubular member, and a corresponding adjustment to the bend angle of the bent housing (because the angular relationship between the first tubular member and the internal tubular member remains constant). Depending on the degree of relative rotation between the inner and second tubular members, the bend angle can be set anywhere between zero degrees and twice the offset angle. For example, if the offset angle is 1.5 degrees, the bent housing assembly's maximum bend angle will be 3.0 degrees.
The preceding discussion describes only one possible configuration for adjustable bent housings. One or more alternative structural configurations could be used to achieve substantially the same functionality.
Adjustable bent housings currently in use employ a variety of different clutch mechanisms to lock the assembly at a desired bend angle. A typical example would be mating clutch mechanisms featuring circumferentially-spaced square or angled teeth formed in the first end of the second tubular member and the second end of the adjustment ring. These teeth act as a clutch in that relative rotation between the first and second tubular members is prevented when the teeth are engaged, but relative rotation is freely enabled when the teeth are disengaged. As components of the bent housing assembly are unthreaded during adjustment of the bend angle, such relative rotation sometimes exceeds what is required to traverse the housing's full range of bend angle settings. This can result in housing components being put out of proper axial alignment after making a bend angle adjustment. If the axial misalignment is great enough, during subsequent re-torquing of the assembly the first end of the first tubular member can come into contact with the splines of the internal tubular member prior to the clutch mechanism becoming fully engaged. This subjects the clutch teeth to the full make-up torque, which can cause physical damage to the teeth. This also results in improper make-up of the connection because the make-up torque is transferred through the clutch teeth rather than pre-loading the internal tubular member and adjustment ring between the first and second tubular members. As is well known in the art, an improperly made-up threaded connection can result in fracture due to excessive bending stresses, as well as the potential for the connection to become unthreaded during operation.
For the foregoing reasons, there is a need for a clutch mechanism for interlocking engagement of tubular components of an adjustable bent housing which prevents relative over-rotation of housing components and resultant physical damage thereto.